1. Field of the Invention
The present invention relates to a method and a device for depositing thin layers by chemical vapor deposition (CVD), possibly by low pressure chemical vapor deposition (LPCVD).
2. Discussion of the Related Art
According to a so-called CVD method, the chemical elements to be deposited are conveyed directly or in the form of gas compounds (precursors) to a deposition chamber where is placed a substrate on which deposition must be achieved. The mixture is introduced into an area where the reaction occurs, activated by an energy input (temperature increase, plasma, photons, and so on), the reaction generally provides only a single solid compound. Gas residues are evacuated outside of the reactor. ##STR1##
The method generally uses as many sources as elements present in the compound to synthesize. One or more carrier gases, which may be neutral or may participate to the reaction, convey the precursor vapors up to the deposition area.
In the CVD method, one of the problems encountered is the introduction of precursors into the deposition chamber.
FIG. 1 schematically illustrates the problem to solve. A chamber 1 is maintained at a temperature and at a pressure adapted to the deposition on a substrate 2, which is generally heated. One or more pipes 3 lead into the airtight chamber for introducing diluent or reactive carrier gases and pipes 4, 5, 6 for introducing gaseous precursors into the airtight chamber.
If precursors are initially gaseous at room temperature, there are few problems. If the precursors are in liquid or solid phase, it is necessary to heat them in tanks 7, 8, 9 to volatilize them and therefore to obtain a sufficient vapor pressure in order to convey the material by a carrier gas.
In the particular case where one or more solid or liquid sources are used, it is necessary to perfectly stabilize the temperature and the flow of the carrier gas of each source to set the composition of the deposition. Thus, for a compound including n elements, the temperature and the flow of each source must be controlled, i.e., 2n parameters. In addition, when a solid in the form of a powder is heated to be vaporized, its evaporation surface may often be blocked by impurities or by oxidation by humidity traces in the carrier gas, which causes the surface to change and the performance to be unsteady. This surface also varies while the solid source is consumed. Furthermore, in the case of solids having very high sublimation temperatures, this temperature must be maintained in the whole pipe between the source and the chamber. This raises several problems, i.e., basic problems as regards the possible decomposition of unsteady precursors in the source or in the path between the source and the deposition chamber, and technical problems for the fabrication of valves adapted to withstand high temperatures.
Several solutions have been proposed in the prior art to overcome these problems.
A first solution consists of regularly taking small amounts of precursor powder in the solid phase and introducing them directly into the evaporator where they are immediately evaporated. This method enables one to obtain a controlled vapor flow even with thermally unsteady precursors because only the small amount of precursor is heated at high temperature in the deposition chamber after introduction into this chamber. However it is clear that the method has the following drawbacks.
It is difficult to accurately measure the injection of regular powder amounts. This requires a perfect control of the granulometry and the fabrication of a device capable of injecting micro amounts of powders (some 10.sup.-6 grams of powder per injection).
The devices for measuring powders are not accurate enough to allow the use of several solid injectors in parallel to synthesize compounds comprising several elements. In order to simultaneously inject several precursors, it is necessary to use mixtures comprising perfectly homogeneous powders (both for the composition and for the particle size distribution) which raises a technical problem, which is difficult to solve.
A second solution consists of using a nebulization method. The liquid or solid precursors that are melt in a solvent, to obtain a liquid source, are turned into aerosols by ultrasonic nebulizers. The aerosol is conveyed up to the deposition area by an active or neutral carrier gas. Various liquid precursors or precursors in solution can be mixed before being transformed into an aerosol. Generally, a single source is used, whereas it is possible to use several aerosol generators in parallel. The method is advantageous over the conventional CVD method in that the concentration ratio of the different elements brought in the deposition area remains constant over time if there is only one source. Only the total flow can vary if the solution viscosity varies. However, it is clear that the solution has the main following drawbacks.
The ultrasonic production of aerosol causes the solution to be heated, and thereby viscosity variation. Therefore, it is very difficult to stabilize the aerosol production (that is the reason why there is generally only one source and, if a device with several sources is possible, it is seldom used). The system is difficultly usable for fabricating alternated multilayers in a single operation.
The ultrasonic excitation required to generate an aerosol may, in some cases, activate a chemical reaction between the precursors and, possibly, the solvent of a solution. This causes a modification of the amounts of precursors brought into the reaction area, which is detrimental for the quality of deposition.
Only liquids or solutions having a low viscosity can be nebulized. This requires the use of very fluid solvents and generally dilute solutions, therefore low partial vapor pressures of the precursors in the substrate area.
Since the solutions used are unavoidably dilute because of viscosity, a large amount of solvent is present in the vapor phase in the reaction area. This important vapor amount of solvent at the substrate may cause spurious reactions that are detrimental for the quality of depositions.
In several cases, the carrier gas cooperates to the reaction (for example, oxygen or air for oxide depositions, ammonia for nitride depositions, hydrogen sulfide for nitride depositions). The mixture of a reactive gas with important amounts of organic solvent may cause explosive reactions in the reactor. The risk of explosion may be limited by using low pressure, but it is then necessary to use a solvent with a low vapor pressure to prevent the sources from boiling. The need for low vapor pressure and low viscosity solvents substantially reduces the selection of usable solvents.